The present invention relates to a signal generating apparatus, and more particularly to a phase-locked loop based transmitter with an open loop modulation compensation scheme, wherein the modulation compensation scheme calibrates a compensation filter according to an output frequency of a controllable oscillator of the signal generating apparatus, and a method thereof.
Please refer to FIG. 1. FIG. 1 is a related art Sigma-delta fractional N phase locked loop (PLL) transmitter 10. The Sigma-delta fractional N PLL transmitter 10 comprises a phase locked loop circuit 11, a Sigma-delta modulator 12, a channel selector 13, a Gaussian filter 14, and a compensation filter 15. The phase locked loop circuit 11 comprises a phase/frequency detector 11a, a charge pump circuit 11b, a loop filter 11c, a voltage-controlled oscillator 11d, and a divider 11e. The baseband data Sb is inputted to the Gaussian filter 14 to extract the required band of the Sigma-delta fractional N PLL transmitter 10. For example, if the transmitted signal of the Sigma-delta fractional N PLL transmitter 10 is the GMSK signal SGMSK, then the Gaussian filter 14 is the GMSK filter. Furthermore, as the frequency response of the phase locked loop circuit 11 acts like a low pass filter, the compensation filter is utilized to compensate the baseband data Sb before being modulated by the Sigma-delta modulator 12. Furthermore, the phase locked loop circuit 11 utilizes a reference frequency Sr to synthesize the required frequency to transmit the baseband data Sb. In a multi-band system, the phase locked loop circuit 11 needs to generate different frequency bands, and the channel selector 13 is utilized to select the required frequency band. Accordingly, by adjusting the dividing number of the divider 11e, the phase locked loop circuit 11 can generate different frequency bands accordingly.
However, it is well-known that the loop bandwidth of the phase locked loop circuit 11 is sensitive to the transfer function of the phase locked loop circuit 11. To deal with this problem, some conventional techniques have been disclosed, such as U.S. Pat. Nos. 7,103,337, 7,068,112, 6,724,265, and No. 6,806,780.